The groups, the Wildlife Trusts and the World Wide Fund for Nature-UK, have published a report, "An Economically Efficient Strategy for Coastal Defence and the Conservation of the Intertidal Zone".

The report says some scientists regard the intertidal zone - the area between high and low water marks - as "more biologically productive than a rainforest".

"In the UK, millions of migrating birds depend on the threatened coastal habitats to survive as they journey from summer breeding grounds to their wintering refuges," it says. "Species that depend on the intertidal habitat include the oystercatcher, knot, Brent goose and redshank.

Pollution filter

The groups make the point that this habitat is also an important source of food for a number of commercial fish species such as plaice and flounder. More than 100 rare plants also said to depend on coastal habitats.

"Coastal wetlands are also highly effective sinks for both marine and river-borne pollution, breaking down pesticides, nutrients, PCBs (polychlorinated biphenyls) and heavy metals."

The report says that unless the concrete coastal defences built in many areas to keep the sea at bay are realigned, 200 square kilometres of habitat will disappear underwater. It says these sea walls, which stretch for more than 2,300 km, "are not allowing the habitat to retreat as sea levels rise".

"As a result, erosion has now replaced development as the primary cause of destruction of the intertidal area."

Economic sense

The two groups want a national strategy to identify where the coastline should be maintained, and where it should be allowed to retreat. They say retreat should be allowed "as a matter of urgency" around the Wash and the Humber and Thames estuaries.

Controversially, the report says that the intertidal zone is itself the most effective form of coastal defence. It cites one calculation showing that "where there is an 80 metre width of saltmarsh in front of a sea wall, the height of the wall needs to be only 3 metres, costing £400,000 per km".

"Without the saltmarsh, the wall would need to be raised to 12 m, which would increase the cost to a staggering £5 million per km."

The report also argues that public money is being seriously misspent. It says the financial information provided by the Environment Agency on coastal defence projects "was completely inadequate to be able to judge whether they were economically justified".

'Managed retreat'

In one case, it says, the Agency cited commercial confidentiality as the reason for not revealing cost-benefit details about a proposal to raise a sea wall around Wallasea Island in Essex.

The report says "further investigation" showed the scheme would protect only one small farm which makes £3,000 a year in profit after deduction of farming subsidies. Yet the scheme would cost taxpayers £2.5 m.

Government departments, including the Ministry of Agriculture, have argued for a strategy of "managed retreat", abandoning some land to the sea rather than trying to protect everything.

It makes sense, especially when farming surpluses mean that not every square inch of land is needed. But the policy continues to fall foul of home owners who see no reason to abandon their houses, which could also be threatened if wildlife habitats take priority.